1. Field of the Invention
The present invention relates to a series of triaminepentaacetic acid compounds, and more particularly to a paramagnetic metal complex prepared from using the compound as the ligand, which can be used as a contrast agent for magnetic resonance imaging (MRI).
2. Description of the Prior Art
In recent years, magnetic resonance imaging (MRI) has developed rapidly and has become one of the most important techniques for diagnosing diseases. In order to increase sensitivity and accuracy, it is very important to develop a safe, stable, and targeting MRI contrast agent. Having a high magnetic moment, paramagnetic metal ions such as Mn.sup.2+, Fe.sup.3+, and Gd.sup.+3 have potential to serve as MRI contrast agents. MRI contrast agents which have been approved by FDA of United States to be intravenously injected clinically include Gd(DTPA).sup.2- (gadopentetate dimeglumine), Gd(DOTA).sup.- (gadoterate megulumine), Gd(DTPA-BMA) (bis-methylamide) (gadodiamide injection), Gd(HP-DO3A) (gadoteridol), and MnDPDP (Teslascan). All of these five contrast agents belong to extracellular agents. Gd(DTPA-BMA) and Gd(HP-DO3A) are nonionic contrast agents, and Gd(DTPA).sup.2-, Gd(DOTA).sup.-, and MnDPDP are ionic ones. Gd(DOTA).sup.- and Gd(HP-DO3A) are macrocyclic, and MnDPDP, Gd(DTPA).sup.2-, and Gd(DTPA-BMA) are open-chained.
Among the above cations, Gd.sup.3+ has the greatest magnetic moment; thus, it has drawn the greatest interest. However, such a cation is not suitable for use alone as a MRI contrast agent due to the reasons of toxicity, pharmacokinetics, biodistribution, and effect. In addition, when GdCl.sub.3 is intravenously injected to animal bodies, the LD.sub.50 is very high, 0.3-0.5 mmol/kg [Weinmann et al. (1984) Am. J. Roentg., 142, 619]. Therefore, it is necessary to use an organic ligand to complex with Gd.sup.3+ to form a stable metal complex to inhibit its toxicity and change the biodistribution and effect. Having a d orbital, Mn.sup.2+ and Fe.sup.3+ can form a stable complex with an organic ligand because of very strong partial covalent bonds [Lauffer et al. (1985) J. Comput. Assist. Tomogr., 9, 431]. It is more difficult for trivalent cations of lanthanide series to form complexes with organic ligands, since the f orbital belongs to the inner orbital which has less orientation and the interaction between the cation and the ligand is totally contributed by electrostatic interaction.
The toxicity of the metal complex can be derived from (1) free metal ion released from dissociation; (2) free organic ligand released from dissociation; and (3) the metal complex itself. In addition, metabolites may be even more toxic than the metal complex itself. Since metal ions and organic ligands may form bonding with proteins, enzymes, or cell membrane in tissues by electrostatic interaction, hydrogen bonds, or covalent bonds, they have higher toxicity than the metal complex itself for animal bodies.
The toxicity of the metal ion is derived from coordination of ions to oxygen, nitrogen, or sulfur atom of macromolecules in animal bodies, thus changing the dynamic equilibria necessary to sustain life. For example, Gd.sup.3+ easily replaces Ca.sup.2+ and binds to Ca.sup.2+ binding sites to complex with the above atoms. The toxicity of the organic ligand is due to the effect to the tissues by the ligand itself. The toxicity of the metal complex may be derived from various reasons. For example, when a large dose of the metal complex is injected, it will cause a difference in osmolality between intracellular and extracellular compartments. Water is drawn out of cells as a result of the osmotic gradient, causing cellular and circulatory damage. Other toxicity reasons include enzyme inhibition or alternation of membrane functions.
To design a new contrast agent for MRI, the stability of the metal complex is the main concern. The contrast agent should be effective during the period of time from injecting it to the body to excreting it from the body. Therefore, stability is required for this residence time. Three factors should be considered to determine the stability of a gadolinium complex in vivo; that is, thermodynamic stability constant, conditional stability constant, and selectivity constant [Cacheris et al. (1990) Magn. Reson. Imag., 8, 467].
Recent reseach on MRI contrast agents can be classified in two categories and are described as follows:
(1) Ionic MRI contrast agents: Such contrast agents include [Gd(DTPA)].sup.2-, [Gd(EOB-DTPA)].sup.2- [ethoxybenzyl diethylene triaminepentaacetate-gadolinium(III)], [Gd(BOPTA)].sup.2- [benzyloxypropionic tetraacetate-gadolinium(III)], and [Gd(DOTA)].sup.-1. The thermodynamic stability constants of [Gd(DTPA)].sup.2-, [Gd(BOPTA)].sup.2- and [Gd(DOTA)].sup.-1 are 10.sup.22.46, 10.sup.22.0, and 10.sup.25.3 [Vittadin et al. (1988) Invest. Radiol., 23, 246; and Pavone et al. (1990) Radiology, 176, 61]. Since these gadolinium complexes are ionic, counter ions, generally meglumine, should be added to form ion pair for storage. However, this will increase the osmotic pressure of the solution.
(2) Non-ionic MRI contrast agents: Such contrast agents have a lower osmotic pressure than ionic contrast agents. Thus, non-ionic MRI contrast agents can be injected in a larger dose in order to achieve higher enhancement effect. Non-ionic MRI contrast agents includes [Gd(DTPA-BMA)] [diethylenetriamine pentaacetic acid bis(methylamide)-gadolinium(III)], [Gd(DTPA-BP)] [N,N-bis(2-pyridylmethyl)diethylenetriamine-N,N',N"-triacetate-gadolinium( III)], [Gd(HP-DO3A)] [10-(2-hydroxypropyl)-1,4,7,10-tetraaza-cyclododecane-1,4,7-triacetate-gad olinium(III)], and [Gd(DO3A)] [1,4,7,10-tetraazacyclododecane-1,4,7-triacetate-gadolinium(III)]. These non-ionic gadolinium complexes have a lower thermodynamic stability than ionic gadolinium complexes. For example, the thermodynamic stability constants of [Gd(DTPA-BMA)], [Gd(DTPA-BP], [Gd(HP-DO3A)] and [Gd(DO3A)] are 10.sup.16.85, 10.sup.16.83, and 10.sup.23.8 and 10.sup.21.0 [Kumar et al. (1994) Inorg. Chem., 33, 3567; and Brucher et al. (1991) Inorg. Chem., 30, 2092]. However, these non-ionic gadolinium complexes are very stable under the physiological conditions of bodies.
The toxicity of the open-chained gadolinium complex, no matter ionic or non-ionic, is mainly related to selectivity constant. For example, the selectivity constant is in the order of [Gd(DTPA-BMA)] (10.sup.9.04)&gt;[Gd(DTPA)].sup.2- (10.sup.7.04)&gt;[Gd(DTPA-BP)] (10.sup.5.32), which is consistent to the order of LD.sub.50, [Gd(DTPA-BMA)] (14.8 mmol/kg)&gt;[Gd(DTPA)].sup.2- (5.6 mmol/kg)&gt;[Gd(DTPA-BP)] (3.2 mmol/kg).
The relaxivity of the metal complex is also an important consideration for designing an MRI contrast agent. When the denticity of the organic ligand increases, the coordinated water molecules in the inner sphere of the metal complex decreas, thus decreasing the relaxation effect. Taking [Gd(EDTA)].sup.- for an example, EDTA has 6 denticities and Gd.sup.3+ has 8-9 binding sites, thus, 2-3 inner sphere water molecules will be present in [Gd(EDTA)].sup.-, and the relaxivity (R.sub.1) is as high as 6.3 (mM s).sup.-1. DTPA, EOB-DTPA, BOPTA, DOTA, HP-DO3A and DTPA-BMA have 8 denticities, thus, only 1 inner sphere water molecule will be present in their gadolinium complex, and the relaxivities are 3.7, 5.3, 5.65, 5.8, 3.7, and 5.1 (mM s).sup.-1 respectively. Gd(III)-TREN-Me-3,2-HOPO [tris(3-hydroxy-1-methyl-2-oxo-1,1-didehydro-pyridine-4-carboxamido)ethyla mine] has 2 inner sphere water 7 molecules, and the relaxivity is increased to 10.5 (mM s).sup.-1 (37.degree. C., 20 MHz) [Xu et al. (1995) J. Am. Chem. Soc., 117, 7245]. Petre et al. modifies DTPA by introducing p-butylbenzyl with higher lipid solubility to the ethyl group of DTPA to form Gd(1RS)-1-(p-butylbenzyl)-DTPA, so as to make the contrast agent absorbed by livers [Petre et al. (1995) Magn. Reson. Med., 35, 532]. Also, Runge et al. synthesizes 2,5-BPA-DO3A and Cy.sub.2 DOTA (biscyclohexyl-DOTA) to increase their lipid solubility [Runge et al. (1996) Invest. Radiol., 31, 11]. Lammers et al. synthesizes N,N"-bis[N-(d-gluco-2,3,4,5,6-pentahydroxyhexyl)carbamoylmethyl]diethylene triamine-N,N',N"-triacetic acid (DTPA-BGLUCA) and N,N"-bis[N-(3-aza-D-galacto-5,6,7,8,9-pentahydroxynonyl)carbamoylmethyl]di ethylenetriamine-N,N',N"-triacetic acid (DTPA-BENGALAA). The purpose of introducing various aminoglucoses is to increase the functional group, so as to increase the outer sphere water molecules, thus increasing relaxivity [Lammers et al. (1997) Inorg. Chem., 36, 2527]. Vauthey et al. synthesizes 2,11-dihydroxy-4,9-dioxa-1,12-bis[1,4,7,10-tetraaza-4,7,10-tris(carboxymet hyl)cyclododecyl]dodecane-gadolinium(III)) {BO[Gd(DO3A)(H.sub.2 O)]2} dimer. Since the dimer has two inner sphere water molecules (q=2), and the rotational time (.tau..sub.r) of the dimer increases twice compared with the monomer, therefore, the relaxivity (R.sub.1) increases from 3.4 (monomer) to 4.61 dm.sup.3 mmol.sup.-1 s.sup.-1 (dimer) [Vauthey et al. (1996) Inorg. Chem., 35, 3375]. Schumann et al. synthesizes 1,4,7,10,13,16,19,22-octaazacyclotetracosane-1,4,7,10,13,16,19,22-octaacet ic acid (H.sub.8 OTEC) and 1,4,7,10,14,17,20,23-octaazacyclohexacosane-1,4,7,10,14,17,20,23-octaaceti c acid (H.sub.8 HEC). The two ligands have 16 coordinating sites and can form a bimetallic complex with lanthanide series metal ion, which have higher water solubility [Schumann et al. (1997) J. Chem. Ber./Recueil, 130, 267]. Aime et al synthesize 3,6,9,15-tetraazabicyclo[9.3. 1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid (PCTA-[12]), 3,6,10,16-tetraazabicyclo[10.3. 1]-hexadeca-1(16),12,14-triene-3,6,10-triacetic acid (PCTA-[13]), and 3,7,11,17-tetraazabicyclo[11.3. 1]heptadeca-1(17),13,15-triene-3,7,11-triacetic acid (PCTA-[14]), all of which contains pyridine and seven coordinating sites [Aime et al. (1997) Inorg. Chem., 36, 2992]. The complex of pyridine-containing ligands and gadolinium ions has two inner sphere water molecules. The relaxivities of PCTA[12], PCTA[13], PCTA[14] are 6.9, 6.3, and 5.9 dm.sup.3 mmol.sup.-1 s.sup.-1 respectively, which are much higher than that of [Gd(DOTA)] (3.56 dm.sup.3 mmol.sup.-1 s.sup.-1) containing one inner sphere water molecule. The stability constants of PCTA[12]-Gd(III) and PCTA[13]-Gd(III) are 20.8 and 19.3 respectively, which are higher than that of [Gd(DTPA-BMA)] (16.85) and lower than those of [Gd(DTPA)].sup.2- (22.4) and [Gd(DOTA)].sup.- (24.6).
There is still a need to develop a better MRI contrast agent.